Editing AUTOEXEC-BAT And CONFIG-SYS To Get As Close As Possible To 640 K Of Free Memory
| Field | IBM PC Compatible Computers |
| Went Obsolete | 1995 |
| Made Obsolete By | Windows 95 |
| Knowledge Assumed | MS-DOS |
| When useful | Playing games, running large applications |
AUTOEXEC.BAT and CONFIG.SYS are configuration files for loading drivers which are needed to run a computer system. These drivers had to be loaded into real memory, taking up valuable space of the 640KB limit.
The 640KB memory limit comes from the fact that 8086 processors (and the compatibility real-mode of 80286 and later processors) can only access 1MB (or 1024KB) memory, within which the upper 384KB is reserved for ROM-BIOS. However, much of that 384KB is left unused.
Tweaking the drivers so that you still had a mouse, soundcard and CD-ROM available was a skill that was essential for playing early PC games (e.g. DOOM). On a computer with 80386 or later processor and more than 1MB of main memory, one trick is to use HIMEM.SYS, which relocates the DOS kernel, internal data structure, and drivers to the first 64KB after the first megabyte of main memory.
Another trick is to use EMM386.EXE, which maps the unused parts of the 384KB ROM-BIOS area into usable memory for drivers and applications. The unused ROM-BIOS space had to be discovered manually, further complicating the skill. The manual process of discovering unused upper memory blocks and editing AUTOEXEC.BAT/CONFIG.SYS to take advantage of both HIMEM.SYS and EMM386.EXE were later automated with the MEMMAKER utility, introduced in MS-DOS 6.0.
These tricks became a desirable skill because DOS lagged behind the improved memory capability of newer processors (namely 80386) for a long time, while applications increasingly demand more memory. After operating systems started to use protected-mode memory as opposed to real-mode memory, the need to get the max out of the first 640 KB of memory faded, except when playing old games.
Although Windows 95, 98, and ME ran in protected mode and were able to provide applications with much more memory than 640KB, it depended on the underlying MS-DOS 7.0 to run DOS mode programs, so it obsoleted the skill but did not obliterate the need. However, Windows NT, which later became Windows 2000 and then Windows XP, no longer used AUTOEXEC.BAT and CONFIG.SYS, thus eliminating both the skill and the need completely. The skill reappeared many years later with the emergence of virtual PCs (DOS emulators and the like), which is used by nostalgic enthusiasts to play old DOS games.
